Flow batteries with an acidic electrolyte is a new technology in the field of electrochemical energy storage. The advantages include high conversion efficiency, flexible system design, large storage capacity, flexibility in site selection, deep discharge, safe and environmentally friendly, and low maintenance cost, etc. It is one of the preferred technologies for the large scale and high efficiency energy storage. In particular, since the vanadium redox flow battery (VRB) has the advantages of high security, strong stability, high efficiency, long operating life (above 15 years) and low cost, etc., it has been considered as the most promising and representative among flow batteries.
The ion exchange membrane is the key component in a flow battery. It separates the positive and negative electrolytes while serves as the conduit for transporting ions between the positive and the negative electrolyte. Therefore, the ion exchange membrane is required to have a high ionic conductivity, ion selectivity and good chemical stability. After evaluating some commercial membranes, Skyllas-kazacos et al. found that with the exception that the comprehensive property of some perfluorosulfonic polymers is stable enough, other membranes (such as Selemion CMV, CMS, AMV, DMV, ASS and DSV etc.) are not stable enough in acid vanadium solution (J. Appl Electrochem, 2004, 34 (2): 137). However, studies found that even though the commercial perfluorosulfonic polymer possesses high mechanical strength and chemical stability, the permeation rate of vanadium ions when used in VRB is high. There is also obvious water migration between the anode and cathode during charge-discharge cycles. Moreover, the complex production technology, severe preparation conditions and high cost in making perfluorosulfonic polymers have severely limited the application and commercialization of VRB.
Since the aromatic polymer ion exchange membrane with nitrogen heterocycles has excellent mechanical stability, thermal stability and chemical stability, it has received wide-spread attention in different fields.